Rickettsial diseases such as those caused by Spotted Fever Group rickettsii (SFGR) and Scrub typhus (ST) are estimated to account for millions of illnesses a year worldwide. SFGR infections affect Veterans and other members of the public throughout the US, and both SFGR and Scrub typhus are continuous threats to active duty warfighters stationed throughout the world. Effective therapy exists, but there is currently no clinically available test capable of yielding reliable diagnostic information within the timeframe required to guide therapeutic decisions. Through this proposal we intend to explore the pathophysiology of natural human infection with SFGR and ST as defined by the host response, and to utilize this information to generate a practical test capable of diagnosing these diseases in the acute setting. Prior published work and our preliminary data suggest that these rickettsial infections trigger conserved inflammatory pathways in human hosts, and it is these unique host responses that we will harness in order to identify patterns of gene expression within circulating white blood cells that identify the presence of these organisms in febrile patients. In order to generate such a ?signature? of infection, this project will utilize a unique combination of two main clinical cohorts. The first cohort is made up of US Veterans with acute rickettsial illness diagnosed here in the US, both previously enrolled patients with acute SFGR infection as well as ongoing enrollment of new cases. The second cohort is made up of a large existing sample set of individuals enrolled from around the world with a variety of acute febrile SFGR infections as well as Scrub typhus, which we have shown mimics the host responses seen in SFGR infection. This combination will give us the ability to generate a rickettsial gene signature that both identifies acute disease in Veterans here in the US as well as having the potential for utility beyond our borders for travelers and active duty military members stationed internationally. We will utilize RNA Sequencing (RNASeq) to identify complex patterns of gene expression in peripheral blood samples from subjects and controls with other types of febrile illnesses (including viral and bacterial infections, both from the US and globally). However, high dimensional RNASeq analysis is not a modality that can produce diagnostic answers in a clinically relevant timeframe. Therefore, we will ?prune? these complex signature(s) down to smaller genomic classifiers appropriate for RT-PCR platforms, as using RT-PCR probes lends itself to both existing clinical laboratory infrastructure as well as to a number of burgeoning point-of-care platforms in development, thus beginning the final steps towards bringing such a test to the point of actual clinical utility. Thus, through this proposal we intend to explore the pathophysiology of natural human infection with SFGR and Scrub typhus as defined by the host response, and to utilize this information to build systems-based models with diagnostic capability. It is therefore our expectation that not only will these experiments provide us with exciting new data about a largely unexplored aspect of the host response to infections with true global impact, but that they will lay the foundation for translating these findings into practical applications with a direct impact on public and Veteran health.